By the time Jeff was elevated into the main event on a permanent basis, he had already conquered the mid-card. There was nothing left for him to do there. A reign with the U.S. Title was almost beneath him. I'm hardly his biggest fan, but the guy was fucking OVER and then some.
Yes. The kids loved him, but he also had his fair share of older fans in the crowds, too. He wasn't exactly Rey Mysterio like you're trying to imply.
Furthermore, I think a lot of credit should go to the way Jeff's character was booked & carefully handled during his build-up to the World Title victory. Freddie Prinze Jr. has been credited with a lot of that - and if that's true - then I think Freddie was a great asset to the creative team. Jeff was absolutely believable as a loose cannon in those few months.
Jeff's series of matches with Triple H, with all the "close call" finishes, felt very much like Sting's clashes with Flair back in the day. It was all about establishing Jeff and making him into a credible threat to the title. It did just that. So, by the time Jeff captured the gold, it was logical. It made sense.
So, no. I don't think Jeff wouldn't have held the title in a different era. It really doesn't matter. For Jeff to fail to get a World Title, you'd have to subtract so many things from the equation. The series of matches with Triple H, the numerous reigns as Intercontinental Champion, the great build-up and booking he received on SmackDown, his unreal level of popularity at that time, etc., etc., etc.